1. Field of the Invention
The present invention relates to a hydraulic machine that has a wheel mounted to move in rotation relative to a stationary structure and about a stationary axis, said wheel being designed to pass a forced flow of water therethrough. Such a forced flow causes the wheel to be driven in rotation when the machine is a turbine. Such a flow results from said rotation when the machine is a pump.
2. Brief Description of the Related Art
Regardless of whether it is a turbine, a pump, or a pump turbine, a hydraulic machine is generally equipped with a device that is more often known as a “labyrinth” and that is designed to limit water leaks that result from the fact that a quantity of the water brought to the vicinity of a wheel tends to leak around said wheel. Such leaks give rise to loss of efficiency of the installation to which the machine belongs, and said labyrinth aims to limit that by defining operating clearances that are relatively narrow between the wheel and a stationary structure of the machine so that the flow path of the leaks is relatively narrow, thereby limiting the leakage flow-rate.
Labyrinths are generally designed in the form of a cylinder concentric with the axis of rotation of the wheel, with one or more stages imparting clearance that is as small as possible. In order to avoid premature wear of the labyrinth while the wheel is rotating, the operating clearance must be sufficient to take account of any radial movements of the wheel at transient speeds and to take account of any expansion of the wheel under the effect of the centrifugal force. In such a situation, the operating clearance of a current labyrinth must be determined by taking account of the most unfavorable operating conditions, namely transient speeds, and by taking account of the radial movements of the wheel. That leads to operating clearances being defined that are relatively large, and hence to non-negligible leakage around the wheel, in particular when the machine is a Francis turbine.
Analogous problems arise with pumps and with turbine pumps.